We will test aspects of an in vitro model system in which normal diploid fibroblasts are thought to age. Our purpose is to decide whether the mechanisms which underlie it are related to those responsible for aging in vivo. We ask if cell divisions are the cause of cell aging, or whether like other cell functions cell division potential is reduced as organisms age chronologically. We will test the possibility that departure of normal diploid fibroblasts from cycle in vitro may not be an "aging" phenomenon, but rather an expected differentiation of a cell type which in the organism unlike the stem cell usually is found uncoupled from cell divisions. Our approach will depend on the use of an interactive system we have developed and will continue to refine, which allows an operator to participate in and guide computer analysis of filmed cell lineage data. We shall continue to develop a more fully automated system of image processing. Using a cell lineage approach enables us to study every cell of a clone and avoids problems of cell heterogeneity encountered in mass cultures. We will identify and quantitate with time lapse microcinematography using high-resolution microscopy, changes in morphology, motility, cell division cycle, and social behavior. Using antibodies against tubulin, myosin, actin, actinin, and tropomyosin, we will characterize filamentous elements and their density in cloned cells and relate the results to the cells' past history. Similar post facto experiments will be carried out with anticollagen antibody. We will also probe post facto, the responses of cells in filmed clones to various growth hormones, to serum step-down and step-up routines, and to wound healing challenges. We will test the idea that aging may be best assessed by looking for diminution of cell type specific functions. For fibroblasts these include motility, synthesis of collagen and wound healing responses. We will test in a new model system in which cells of any PDL are kept at G but are not in contact with each other various biosynthetic functions which distinguish heterogenous cultures of cells of different PDLs.